Programmable Overcurrent Protection for a Switch

ABSTRACT

Embodiments of the disclosure include a switch having an on-state resistance that varies based on a temperature coefficient of the switch and an overcurrent protection circuit coupled to the switch and having an adjustable overcurrent threshold level determined based on an adjustable voltage generated by the overcurrent protection circuit, the adjustable voltage generated based on the temperature coefficient of the switch.

BACKGROUND

A synchronous switch mode power supply, such as a buck converter, is anelectronic power supply that efficiently converts power from a firstpower regime to a second power regime. Such converters typicallyincorporate a high-side switch (e.g., a “control” switch), a low-sideswitch (e.g., a “synchronous” switch), and an inductor. The inductorcouples a common node (e.g., a “phase node”) of the switches to a loadof the converter. In some applications, the switches are field-effecttransistors (FETs). The high-side switch delivers power to the loadthrough the inductor, thereby converting an input voltage at a firstlevel to an output voltage at a second level. In synchronous buck DC-DCapplications, when an output short condition occurs, current through theinductor increases to maintain the output voltage level at the inductor.However, this approach may cause the inductor to undesirably saturate,which may damage the FET.

SUMMARY

In some embodiments, an overcurrent protection circuit is coupled to aswitch having an on-state resistance that varies based on a temperaturecoefficient of the switch. The overcurrent protection circuit has anadjustable overcurrent threshold level determined based on an adjustablevoltage generated by the overcurrent protection circuit. The adjustablevoltage is generated based on the temperature coefficient of the switch.

In some embodiments, a method for overcurrent protection involvesgenerating, in an overcurrent protection circuit coupled to a switch, anadjustable voltage based on a temperature coefficient of the switch, thegenerated adjustable voltage having a positive temperature coefficient.The method further involves determining an overcurrent threshold levelin the overcurrent protection circuit based on the generated adjustablevoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of an overcurrent protection circuit inaccordance with one or more example embodiments.

FIG. 2 illustrates details of the overcurrent protection circuit of FIG.1, in accordance with some embodiments.

FIG. 3 illustrates further details of the overcurrent protection circuitof FIG. 1, in accordance with some embodiments.

FIG. 4 is an example operation of the overcurrent protection circuit ofFIG. 1, in accordance with some embodiments.

DETAILED DESCRIPTION

Improved methods and circuits are described herein for a programmableovercurrent protection circuit for one or more switches, such asswitches used in a switch-mode power supply (SMPS) circuit having aninductor and one or more field-effect transistor (FET) switches, such asa metal-oxide-semiconductor field-effect transistor (MOSFET). When anoutput short condition occurs at a load of the SMPS circuit, anovercurrent event may occur. An overcurrent event may cause currentthrough the inductor to increase so to maintain the output voltage. Suchan overcurrent event may damage one or more of the FETs of the SMPScircuit.

Some overcurrent protection circuits sense a source-drain current levelthrough a FET of the SMPS circuit and compare the sensed current levelto an overcurrent threshold. If the sensed current level surpasses thecurrent threshold, one or more FETs of the SMPS circuit are turned offto stop current flow through that FET switch to the load of the SMPScircuit.

Current flow through a FET switch is in part related to an on-resistance(Rdson) of a conduction channel formed between a drain region and asource region of that FET. However, the FET on-resistance variesaccording to a temperature coefficient of resistance (TCR) of the FET asa temperature of the FET varies. As such, at lower temperatures, asensed source-drain current level through the FET of the SMPS circuit isthe same or similar to a current level through the load of the SMPScircuit. However, at higher temperature levels, on-resistance Rdson ofthe FET varies according to the TCR of the FET. As temperatures vary, asensed source-drain current level through the FET may diverge from thecurrent level through the load of the SMPS circuit. Thus, a particularovercurrent threshold which is used to accurately determine anovercurrent event at a first temperature of the FET may no longeraccurately determine the overcurrent event at another temperature.

Described herein is a circuit configured for high precision overcurrentprotection that advantageously varies an overcurrent threshold levelaccording to an adjustable temperature coefficient of the circuit astemperature of the circuit varies. The adjustable temperaturecoefficient of the circuit is adjusted to substantially match thetemperature coefficient of resistance of a FET of the circuit. As aresult, as the on-resistance of the FET varies with temperature, theovercurrent threshold will also vary proportionally. Thus, the circuitadvantageously detects an overcurrent condition of the FET across arange of temperatures. Other improvements or advantages will also bedescribed below or become apparent from the following disclosure.

FIG. 1 is a simplified schematic of an overcurrent protection circuit110, in accordance with one or more example embodiments. As shown inFIG. 1, the overcurrent protection circuit 110 is coupled to a switchM2, such as a MOSFET, having an on-resistance Rdson. The on-resistanceRdson varies with temperature based on a temperature coefficient of theswitch M2. In the example embodiment of FIG. 1, the switch M2 is alow-side switch (e.g., a synchronous switch) in a switch-mode powersupply (SMPS) circuit 100, such as a DC-DC buck converter. The switch M2is coupled to and controlled by the overcurrent protection circuit 110.A high-side switch M1 and the switch M2 of the SMPS 100 provide acurrent I_(L) via inductor L1 to an output node Vout. A load (not shown)is typically coupled to the output node Vout. A first terminal of anoutput capacitor Cout, having a resistance symbolically shown byresistor ESR, is coupled at one end to the inductor L1 and the Vout nodeat node 102. A second terminal of the output capacitor Cout is coupledto a voltage ground (PGND). A node 101, sometimes referred to as a phasenode or switch node, is coupled to a port of the overcurrent protectioncircuit 110 designated as SW. The overcurrent protection circuit 110 isconfigured to receive, sense, or measure a voltage and/or current fromthe node 101.

As shown in FIG. 1, the overcurrent protection circuit 110 is coupled toa high gate node (HG) of the switch M1 and to a low gate node (LG) ofthe switch M2. A control logic and driver circuit 105 of the overcurrentprotection circuit 110 controls the turning ON or OFF of the switchesM1, M2 by applying driving signal(s) to the gate(s) of one or both ofthe switches M1, M2. During an overcurrent event, excess amounts ofcurrent I_(L) flows through L1, which can saturate L1 and damage one orboth of the switches M1 and/or M2.

As described below and in greater detail in conjunction with FIGS. 2-4,to protect the SMPS 100, and in particular, the switch M2, from anovercurrent condition, a level of a current or voltage which isproportional to the output current I_(L) is compared to a thresholdcurrent level. The threshold current level can be embodied as a voltageor as a current. If the level of the current or voltage which isproportional to the output current I_(L) exceeds the threshold currentlevel, the overcurrent protection circuit 110 turns one or both of theswitches M1, M2 OFF.

The overcurrent protection circuit 110 is configured to generate anadjustable overcurrent threshold level. As mentioned previously, theovercurrent protection circuit 110 advantageously adjusts theovercurrent threshold level with temperature according to a temperaturecoefficient which is the same or similar to a TCR of the switch M2. Theadjustable overcurrent threshold level is determined based on anadjustable voltage level Vadj which is generated by an adjustablevoltage generator circuit 112. The voltage level Vadj is generated atleast in part based on (e.g., matched or proportionally to) thetemperature coefficient of the switch M2, and a resistive value of aprogrammable scaling resistor Rcs coupled to the overcurrent protectioncircuit 110, as further described below in conjunction with FIG. 2.

As shown in FIG. 1, the adjustable voltage generator circuit 112includes a voltage-to-current generator circuit 114, and acurrent-to-voltage circuit 115, amongst other components, whichcontribute to generating the voltage level Vadj. The generated voltagelevel Vadj is provided to the detection circuit 113 of the overcurrentprotection circuit 110. As described in greater detail below inconjunction with FIG. 3, the detection circuit 113, having aresistor-divider network 117, uses the received voltage level Vadj togenerate the adjustable overcurrent threshold level which is compared toa voltage or current which is proportional or otherwise representativeof the current I_(L).

FIG. 2 illustrates details of the adjustable voltage generator circuit112 of the overcurrent protection circuit 110 shown in FIG. 1. Theadjustable voltage generator circuit 112 is configured to generate thevoltage level Vadj based on (e.g., matched or proportionally to) thetemperature coefficient of the switch M2. In an example embodiment, thevoltage level Vadj has a positive temperature coefficient that isadjusted to substantially match the temperature coefficient of theon-resistance Rdson of the switch M2. As shown in FIG. 2, the adjustablevoltage generator circuit 112 includes the voltage-to-current generatorcircuit 114, which includes a switch T1, such as a bipolar junctiontransistor (BJT) having a base, an emitter and a collector. In someembodiments, the voltage-to-current generator circuit 114 has a negativetemperature coefficient (e.g., Vbe of T1 changes at a rate ofapproximately −2 mV/deg C.) and is configured to generate an outputvoltage V1 at node 201 based on a reference voltage Vref received at thebase of T1. V1 is substantially equal to Vref minus the voltage Vbebetween the base and emitter of T1. In some embodiments, Vref ispre-adjusted to substantially match the temperature coefficient of theon-resistance Rdson of the switch M2. In yet other embodiments, for adifferent temperature coefficient of the on-resistance Rdson of theswitch M2, Vref is trimmed such that the Vref−Vbe matches thetemperature coefficient of the Rdson of the switch M2.

The adjustable voltage generator circuit 112 further includes thecurrent-to-voltage circuit 115 coupled to the voltage-to-currentgenerator circuit 114 to receive the voltage V1 at node 201. In someembodiments, the current-to-voltage circuit 115 has a net positivetemperature coefficient and is configured to generate, based on thevoltage V1, an output voltage V_(ILIM). In other embodiments, thevoltage-to-current generator circuit 114 has a positive temperaturecoefficient and the current-to-voltage circuit 115 has a negativetemperature coefficient.

In the example embodiment of FIG. 2, the current-to-voltage circuit 115is implemented using a resistor-divider configuration with a resistor ROwhich receives voltage V1 from node 201, and a variable resistor R1coupled in series to resistor RO at a common node 202. The voltageV_(ILIM) is thus generated based on the following Equation 1:

$\begin{matrix}{V_{ILIM} = \frac{R\; 1*\left( {{Vref} - {Vbe}} \right)}{\left( {{R\; 1} + {R\; 0}} \right)}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

The generated voltage V_(ILIM) is output from the common node 202 to anon-inverting (+) input of Op-Amp 203, which in turn drives a gate of aswitch M3, such as a MOSFET. The switch M3 is coupled at a source nodeto both the inverting (−) input of Op-Amp 203 and to a scaling resistorRcs. The scaling resistor Rcs is coupled to the adjustable voltagegenerator circuit 112 between ports I_(ILM) and ground (AGND), as shownin FIG. 2. In an example embodiment, the scaling resistor Rcs is aprogrammable overcurrent resistor having a selected (i.e., programmed)value which is used in determining the overcurrent threshold level ofthe overcurrent protection circuit 110.

As also shown in FIG. 2, the adjustable voltage generator circuit 112further includes a current mirror 204 circuit configured to output athreshold current I_(threshold) based on the voltage V_(ILIM) and avalue of the scaling resistor Rcs. The adjustable voltage generatorcircuit 112 also includes a trim resistor Rztc coupled at a firstterminal to the current mirror 204 via node 205, and to a ground node(AGND) at a second terminal. In an example embodiment, the trim resistorRztc is a variable resistor having a temperature coefficientsubstantially equal to zero. The adjustable voltage generator circuit112 generates the voltage level Vadj based on the threshold currentI_(threshold) and a value of trim resistor Rztc, and outputs the voltagelevel Vadj from node 205.

The voltage level Vadj varies with temperature in accordance with apositive temperature coefficient which substantially matches thetemperature coefficient of the on-resistance Rdson of the switch M2based on the following Equation 2:

$\begin{matrix}{{Vadj} = \frac{V_{ILIM}*{Rztc}}{Rcs}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Substituting V_(ILIM) from Equation 1 into Equation 2 results in thefollowing Equation 3:

$\begin{matrix}{{Vadj} = \frac{R\; 1*\left( {{Vref} - {Vbe}} \right)*{Rztc}}{\left( {{R\; 0} + {R\; 1}} \right)*{Rcs}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

Thus, as shown by Equations 1-3, the voltage level Vadj and its positivetemperature coefficient is generated based on (a) a temperaturecoefficient and base voltage (e.g., Vref) of the switch T1 (whichgenerates Vref−Vbe), (b) respective values of the resistor RO and thevariable resistor R1 (which generate voltage V_(ILIM) from Vref−Vbe),(c) a value of the programmable scaling resistor Rcs, and (d) a value ofvariable trim resistor Rztc.

FIG. 3 illustrates details of the detection circuit 113 of theovercurrent protection circuit 110 shown in FIG. 1. As shown in FIG. 3,the detection circuit 113 is coupled to and configured to receive thevoltage level Vadj from the adjustable voltage generator circuit 112.The detection circuit 113 is also coupled to the SMPS 100 via the highgate node (HG) of the high-side switch M1 and the low gate node (LG) ofthe low-side switch M2. During an overcurrent event, an overcurrentevent detection signal Voc is received at the control logic and drivercircuit 105 of the detection circuit 113. Upon receiving the overcurrentevent detection signal Voc, the control logic and driver circuit 105reacts by turning one or both of the switches M1, M2 off to end theovercurrent event. The overcurrent event detection signal is based on ascaled level of the received voltage level Vadj, a voltage at the phasenode 101, and a reference voltage (e.g., ground).

As shown in FIG. 3, the voltage level Vadj generated by the adjustablevoltage generator circuit 112, is received in a non-inverting (+) inputof operational amplifier Op-Amp 301 of the detection circuit 113. Basedon the received voltage level Vadj, the Op-Amp 301 is configured togenerate a voltage V3 at node 302. In an example embodiment, the voltageV3 is substantially equal to the voltage level Vadj.

Node 302 is coupled to an input of the resistor-divider network 117. Inan example embodiment, the resistor-divider network 117 includesseries-connected resistors kR and R2 coupled at a common node 303. Theresistor-divider network 117 is configured to receive the generatedvoltage V3 at a first terminal and a voltage/current of the phase node101 at a second terminal. Based on the received voltage V3 and thevoltage/current of the phase node 101, the resistor-divider network 117generates a voltage V_(OCN) which is provided to a non-inverting (+)input of Overcurrent (OC) Comparator 304. The OC Comparator 304 comparesthe voltage V_(OCN) to a reference voltage (e.g., ground, or anotherbias voltage) coupled to the inverting (−) input of the OC Comparator304 and outputs an overcurrent event detection signal Voc. Theovercurrent event detection signal Voc is then provided to the controllogic and driver circuit 105. Thus, the adjustment to overcurrentthreshold level is advantageously made such that the overcurrentthreshold level varies in substantially the same way that Rdson variesacross a range of temperatures.

In some embodiments, a high output state of the OC Comparator 304indicates an overcurrent event, and a low output state of the OCComparator 304 indicates the absence of an overcurrent event. In someembodiments, while the output state of the OC Comparator 304 is low, thecontrol logic and driver circuit 105 cycles the switch M2 between ON andOFF states. At the moment when V_(OCN)=V_(OCP) the output state of theOC Comparator 304 is high. In some embodiments, upon receiving a highoutput state from the OC Comparator 304, the control logic and drivercircuit 105 turns one or both of the switches M1, M2 OFF. WhenV_(OCN)=V_(OCP):

$\begin{matrix}{{Vsw} = \frac{V\; 3}{k}} & \left( {{Equation}\mspace{14mu} 4} \right) \\{{Vsw} = {{Ioc}*{{Rdson}.}}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

where I_(OC) is the overcurrent threshold level of the overcurrentprotection circuit 110.

As previously stated, V3 is equal, or substantially equal, to thevoltage level Vadj determined in Equation 3 above. Substituting thevoltage level Vadj for V3 in Equation 5 leads to the following Equation6:

$\begin{matrix}{{Ioc} = {\frac{Vsw}{Rdson} = {\frac{V\; 3}{k*{Rdson}} = {\frac{\left( {{Vref} - {Vbe}} \right)}{Rdson}*\frac{R\; 1*{Rztc}}{k*R\; 0*{Rcs}}}}}} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

The voltage Vref can be trimmed to make Vref−Vbe match the temperaturecoefficient of the on-resistance Rdson of the switch M2. Thus, theovercurrent threshold I_(OC) changes proportionally to a current throughM2 as the temperature changes in the SMPS 100. A trim resistor Rztc isutilized in some embodiments to trim or otherwise compensate for processvariations which may occur during manufacturing of the overcurrentprotection circuit 110. In some embodiments, the trim resistor Rztc is aresistor circuit having a TCR substantially equal to zero. An advantageof the above approach is that the I_(OC) will have little or notemperature dependency and therefore its value can be more accuratelydetermined.

FIG. 4 illustrates an example operation of the overcurrent protectioncircuit 110. The process begins at Block 410 in which an adjustablevoltage level Vadj is generated based on a temperature coefficient ofthe switch M2, with the generated voltage level Vadj having a netpositive temperature coefficient. Next, at Block 420, an overcurrentthreshold level is determined by the overcurrent protection circuit 110based on the generated voltage level Vadj. Operations in Blocks 410 and420 are performed in a manner consistent with those described above indetail in conjunction with FIGS. 1-3.

Reference has been made in detail to embodiments of the disclosedinvention, one or more examples of which have been illustrated in theaccompanying figures. Each example has been provided by way ofexplanation of the present technology, not as a limitation of thepresent technology. In fact, while the specification has been describedin detail with respect to specific embodiments of the invention, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing, may readily conceive of alterations to,variations of, and equivalents to these embodiments. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers all such modificationsand variations within the scope of the appended claims and theirequivalents. These and other modifications and variations to the presentinvention may be practiced by those of ordinary skill in the art,without departing from the scope of the present invention, which is moreparticularly set forth in the appended claims. Furthermore, those ofordinary skill in the art will appreciate that the foregoing descriptionis by way of example only and is not intended to limit the invention.

What is claimed is:
 1. An apparatus comprising: a switch having anon-state resistance that varies based on a temperature coefficient ofthe switch; and an overcurrent protection circuit coupled to the switchand having an adjustable overcurrent threshold level determined based onan adjustable voltage generated by the overcurrent protection circuit,the adjustable voltage generated based on the temperature coefficient ofthe switch.
 2. The apparatus of claim 1, further comprising: aswitch-mode power supply circuit coupled to and controlled by theovercurrent protection circuit, wherein the switch-mode power supplycircuit includes the switch.
 3. The apparatus of claim 2, wherein theadjustable voltage has an adjustable temperature coefficient, andwherein the overcurrent protection circuit is configured to adjust theadjustable temperature coefficient to substantially match thetemperature coefficient of the switch.
 4. The apparatus of claim 1,further comprising: a programmable scaling resistor coupled to theovercurrent protection circuit, wherein the adjustable voltage isdetermined based on a value of the programmable scaling resistor.
 5. Theapparatus of claim 2, wherein the overcurrent protection circuit furthercomprises: an adjustable voltage generator circuit configured togenerate the adjustable voltage based on the temperature coefficient ofthe switch; and a detection circuit coupled to the adjustable voltagegenerator circuit and to the switch-mode power supply circuit, thedetection circuit configured to adjust a current supplied to theswitch-mode power supply circuit based on (a) the adjustable voltage,and (b) a voltage corresponding to the switch received from theswitch-mode power supply circuit.
 6. The apparatus of claim 5, whereinthe adjustable voltage generator circuit comprises: a voltage-to-currentgenerator circuit having a negative temperature coefficient andconfigured to generate a first voltage based on a received referencevoltage; and a current-to-voltage circuit coupled to thevoltage-to-current generator circuit, the current-to-voltage circuithaving a positive temperature coefficient and configured to generate,based on the first voltage, a second voltage having a net positivetemperature coefficient.
 7. The apparatus of claim 6, wherein thevoltage-to-current generator circuit includes a bipolar junctiontransistor (BJT) having a base, an emitter and a collector, wherein thereceived reference voltage is received at the base of the BJT.
 8. Theapparatus of claim 6, wherein the received reference voltagesubstantially matches the temperature coefficient of the switch.
 9. Theapparatus of claim 6, wherein the current-to-voltage circuit comprises:a first resistor coupled to the voltage-to-current generator circuitconfigured to receive the first voltage; and a variable resistor coupledin series to the first resistor via a common node, wherein the secondvoltage is outputted from the common node.
 10. The apparatus of claim 6,wherein the adjustable voltage generator circuit further comprises: acurrent mirror circuit configured to output a threshold current based on(a) the second voltage and (b) a value of a scaling resistor coupled tothe overcurrent protection circuit.
 11. The apparatus of claim 10,wherein the adjustable voltage generator circuit further comprises: atrim resistor coupled to the current mirror circuit, wherein theadjustable voltage generator circuit is configured to generate theadjustable voltage based on the threshold current and the trim resistor.12. The apparatus of claim 11, wherein the trim resistor has atemperature coefficient substantially equal to zero.
 13. The apparatusof claim 11, wherein the trim resistor is a variable resistor.
 14. Theapparatus of claim 5, wherein the detection circuit comprises: anoperational amplifier configured to generate a third voltage based onthe adjustable voltage received from the adjustable voltage generatorcircuit; and a resistor-divider network comprising a plurality ofseries-connected resistors configured to receive the third voltage andthe voltage corresponding to the switch received from the switch-modepower supply circuit; wherein the adjustable overcurrent threshold levelis based on an output voltage of the resistor-divider network.
 15. Theapparatus of claim 14, wherein the third voltage is substantially equalto the adjustable voltage.
 16. The apparatus of claim 5, wherein theovercurrent protection circuit is configured to adjust the currentsupplied to the switch-mode power supply circuit independently oftemperature changes in the switch-mode power supply circuit.
 17. Amethod comprising: generating, in an overcurrent protection circuitcoupled to a switch, an adjustable voltage based on a temperaturecoefficient of the switch, the adjustable voltage having a positivetemperature coefficient; and determining an overcurrent threshold levelin the overcurrent protection circuit based on the adjustable voltage.